A Multiple Input Multiple Output (MIMO) scheme refers to a scheme for improving data transmission/reception efficiency using multiple transmit antennas and multiple receive antennas, unlike a scheme using one transmit antenna and one receive antenna. A receiver receives data through multiple paths when multiple antennas are used, whereas the receiver receives data through a single antenna path when a single antenna is used. Accordingly, MIMO can increase a data transmission rate and the amount of transmission data, and improve coverage.
In a MIMO wireless communication system, a receiver (e.g., a user equipment) may feed back a variety of feedback information (for example, a Rank Indicator (RI), a Precoding Matrix Index (PMI), and Channel Quality Information (CQI), etc.) to a transmitter (e.g., a base station). The feedback information may also be referred to as channel state information (CSI). The MS may feed back preferred PMI and RI to the BS on the basis of channel measurement result received from the BS. The MS-preferred RI corresponds to a downlink transmission rank value having the highest transfer rate on the assumption that the MS preferred RI is used by the BS in a given channel state. In addition, the MS-preferred PMI is an index indicating a precoding matrix appropriate for a channel state measured by the MS from among a codebook indicating a set or aggregate of candidate precoding matrices, and the codebook is predefined and shared between the BS and the MS. CQI is calculated on the basis of PMI/RI reported by the MS, and corresponds to a Modulation and Coding Scheme (MCS) level applied to downlink transmission. In this case, CQI may represent an MCS level providing an allowable packet error rate when a rank value and precoder information based on the MS reported PMI/RI.